1. Field of the Invention
This invention relates to pressurized water reactor systems and, more particularly, to a vent system and method of operation for controlling and actuating hydraulically operated displacer rod drive mechanisms for selective positioning of water displacer rods in the reactor vessel.
2. State of the Relevant Art
As is well known in the art, conventional pressurized water reactors employ a number of control rods which are mounted within the reactor vessel, generally in parallel axial relationship, for axial translational movement in telescoping relationship with the fuel rod assemblies. The control rods contain materials known as poisons, which absorb neutrons and thereby lower the neutron flux level within the core. Adjusting the positions of the control rods relative to the respectively associated fuel rod assemblies thereby controls and regulates the reactivity and correspondingly the power output level of the reactor.
Typically, the control rods, or rodlets, are arranged in clusters, and the rods of each cluster are mounted to a common, respectively associated spider. Each spider, in turn, is connected to a respectively associated adjustment mechanism for raising or lowering the associated rod cluster.
In certain advanced designs of such pressurized water reactors, there are employed both reactor control rod clusters (RCC) and water displacer rod clusters (WDRC). In one such reactor design, a total of over 2800 reactor control rods and water displacer rods are arranged in 185 clusters, each of the rod clusters being mounted to a respectively corresponding spider. In the exemplary such advanced design pressurized water reactor, there are provided, at successsively higher, axially aligned elevations within the reactor pressure vessel, a lower barrel assembly, an inner barrel assembly, and a calandria, each of generally cylindrical configuration, and an upper closure dome, or head. The lower barrel assembly may be conventional, having mounted therein, in parallel axial relationship, a plurality of fuel rod assemblies which are supported at the lower and upper ends thereof, respectively, by corresponding lower and upper core plates. Within the inner barrel assembly there is provided a large number of rod guides disposed in closely spaced relationship, in an array extending substantially throughout the cross-sectional area of the inner barrel assembly. The rod guides are of first and second types, respectively housing therewithin reactor control rod clusters (RCC) and water displacer rod clusters (WDRC); these clusters, as received in telescoping relationship within their respectively associated guides, generally are aligned with respectively associated fuel rod assemblies.
One of the main objectives of the advanced design, pressurized water reactors to which the present invention is directed, is to achieve a significant improvement in the fuel utilization efficiency, resulting in lower, overall fuel costs. Consistent with this objective, the water displacement rodlet clusters (WDRC's) function as a mechanical moderator control, all of the WDRC's being fully inserted into association with the fuel rod assemblies, and thus into the reactor core, when initiating a new fuel cycle. Typically, a fuel cycle is of approximately 18 months, following which the fuel must be replaced. As the excess reactivity level diminishes over the cycle, the WDRC's are progressively, in groups, withdrawn from the core so as to enable the reactor to maintain the same reactivity level, even though the reactivity level of the fuel rod assemblies is reducing due to dissipation over time. Conversely, the control rod clusters are moved, again in axial translation and thus telescoping relationship relatively to the respectively associated fuel rod assemblies, for control of the reactivity and correspondingly the power output level of the reactor on a continuing basis, for example in response to load demands, in a manner analogous to conventional reactor control operations.
The calandria includes a lower calandria plate and an upper calandria plate. The rod guides are secured in position at the lower and upper ends thereof, respectively, to the upper core plate and the lower calandria plate. Within the calandria and extending between the lower and upper plates thereof is mounted a plurality of calandria tubes in parallel axial relationship, respectively aligned with the rod guides. Flow holes are provided in remaining portions of the calandria plates, intermediate the calandria tubes, through which passes the reactor core outlet flow as it exits from its upward passage through the inner barrel assembly. The core outlet flow, or a major portion thereof, turns from the axial flow direction to a radial direction for passage through radially outwardly oriented outlet nozzles which are in fluid communication with the calandria.
In similar, parallel axial and aligned relationship, the calandria tubes are joined to corresponding flow shrouds which extend to a predetermined elevation within the head, and which in turn are connected to corresponding head extensions which pass through the structural wall of the head and carry, on their free ends at the exterior of and vertically above the head, corresponding adjustment mechanisms, as above noted. The adjustment mechanisms have corresponding control shafts, or drive rods, which extend through the respective head extensions, flow shrouds, and calandria tubes and are connected to the respectively associated spiders mounting the clusters of RCC rods and WDRC rods, and serve to adjust their elevational positions within the inner barrel assembly and, correspondingly, the level to which the rods are lowered into the lower barrel assembly and thus into association with the fuel rod assemblies therein, thereby to control the activity within the core.
In the exemplary, advanced design pressurized water reactor, over 2,800 rods are mounted in 185 clusters, the latter being received within corresponding 185 rod guides. Of these clusters, 88 are of the WDRC type, divided into 22 groups of four clusters each, the clusters of each group being chosen such that withdrawal of an individual group, or multiple such groups, maintains a symmetrical power distribution within the reactor core. Since each WDRC is approximately 700 lbs. to 800 lbs. in weight, each group of four (4) such clusters presents a combined weight of in the range of from 2,800 lbs. to 3,200 lbs., requiring that a drive mechanism and associated connecting structure for each group of four clusters have substantial strength and durability, and afford a substantial driving force.
Due to the packing density, or close spacing, of the rod clusters and their associated guides, severe spacing requirements are imposed, both within the vessel and with respect to the rod drive mechanisms, including both the water displacer rod drive mechanisms (DRDM's) and the control rod drive mechanism (CRDM's). The critical spacing requirements were not experienced in reactors of prior, conventional types, which did not employ WDRC's and correspondingly did not employ DRDM's. In reactors of such conventional designs, ample spacing was available above the dome, or head, of the vessel for accommodating the required number of mechanisms for driving the RCC's. Particularly, the CRDM's of well known, electromechanical type associated with corresponding clusters of RCC's, were mounted in generally parallel axial relationship, vertically above the dome or head of the vessel and extended in sealed relationship through the head for connection by suitable drive rods to the associated RCC's, and provided for selectively controlled gradual raising and lowering of the RCC's for moderating the reactor energy level, or for rapidly lowering same in the case of shutdown requirements.
In reactor systems of the advanced design herein contemplated, whereas the same mechanisms conventionally employed for the CRDM's functionally are acceptable for adjusting the WDRC's, due to the increased number of rod clusters (i.e., the total of RCC's and WDRC's), the conventional CRDM's are unacceptable mechanically, since they are too large. Various alternative mechanisms have been studied in view of this problem. For example, roller nut-drives were considered, but were determined to produce insufficient lifting force.
Accordingly, a substitute DRDM has been developed which utilizes a hydraulically operated piston which is attached through a corresponding drive rod to each group of associated WDRC's, and which mechanism satisfies the spacing limitations, permitting mounting thereof above the head or dome of the vessel in conjunction with the conventional CRDM's. An example of such a hydraulically operated drive mechanism for a WDRC is shown in U.S. Pat. No. 4,439,054--Veronesi, issued Mar. 27, 1984 and assigned to the common assignee hereof.
The provision of the hydraulically operated mechanism suitable for use with WDRC's as hereinabove set forth, however, has imposed a design requirement of a system to control and manipulate the hydraulic mechanism. No known systems are available for this purpose, in view of the fact that the requirement therefor has arisen out of the evolving design of the advanced design, pressurized water reactors of the type herein contemplated.